This invention relates generally to a method for treating metal surfaces to reduce the formation of carbonaceous deposits, or coke, on furnace or heat exchanger tubes that are exposed to process streams containing hydrocarbons at high temperature. The invention also relates to use of particular antifoulants that are characterized as reducing the rate of coke formation when applied using such treatment methods.
Many refining and petrochemical processes, such as thermal cracking, heavy oil upgrading, and delayed coking, operate under conditions of high temperature and hydrocarbon partial pressure, leading to the formation of coke deposits on critical heat transfer surfaces. These coke deposits act as thermal insulators to substantially increase furnace tube wall temperature. This rise in tube wall temperature reduces heat transfer efficiency, process throughput capacity, and equipment life, and results in increased energy consumption. The coke deposits also result in embrittlement of the tube wall due to carburization of the tube metallurgy.
In one example, ethylene is produced by the thermal cracking of hydrocarbon reactants contained inside of a heated furnace tube. Coke forms on the internal surfaces of the furnace tubes as a by-product of the cracking reactions. As the coke layer grows, the furnace tube wall temperatures rise about 100xc2x0 C. and furnace fuel consumption increases by 5%. The coke layer also increases the furnace pressure drop, thereby reducing product yield and throughput. When tube wall temperatures approach their design limit, or pressure drops becomes excessive, the furnace tubes must be taken off-line and decoked. For ethane crackers, decoking is commonly required at regular intervals of about 45-60 days. The presence of coke promotes carburization of the tube wall, which in combination with increased tube wall temperature, significantly shortens furnace tube operating life.
Coke growth on clean tube surfaces is catalyzed by the metal constituents, primarily Fe and Ni, contained in the tube alloy. Catalytic coke consists of hollow filaments that contain metal-microparticles located at the filament tips. The accepted mechanism for filament growth involves the catalytic decomposition of hydrocarbons at the metal tip, leading to the formation of atomic carbon which diffuses through the metal and deposits on the opposing side. Because the active metal is carried along at the tip of the growing fiber, the catalytic influence of the substrate is maintained over a period of time. Because the initial coke growth rate is surface dependent, there is an opportunity to eliminate catalytic coke deposition by modifying the chemical nature of the tube surface.
A variety of methods have been proposed to reduce the rate of coke formation in cracking processes. U.S. Pat. Nos. 5,616,236 and 5,565,087 describe use of certain tin and silicon antifoulants in the presence of a reducing gas and certain sulfur compounds. U.S. Pat. No. 5,575,902 describes use of a Group VIB metal protective layer that is anchored to the steel tube through an intermediate carbide-rich bonding layer. U.S. Pat. No. 5,242,574 describes use of metal oxide, metal carbide, metal nitride and metal silicide coatings. U.S. Pat. No. 5,015,358 describes use of certain tin, chromium, and antimony antifoulants. U.S. Pat. Nos. 4,692,313 and 4,454,021 describe use of alkali and alkaline earth inhibitors. U.S. Pat. No. 4,410,418 describes use of certain silicon antifoulants.
Methods described in the prior art for application of antifoulants and coke inhibitors to metal surfaces include electrochemical deposition, chemical vapor deposition; plasma-assisted deposition and thermal diffusion processes. A common drawback of deposition methods is that the treated layer is susceptible to cracking, peeling, and degradation in high temperature thermal cracking processes. It is well known that a mismatch in thermal expansion coefficients between the deposited layer and the metal substrate results in severe mechanical stresses in the coating layer. Furthermore, deposition methods can result in coatings that are not uniformly distributed on the substrate surface or possess microscale defects that increase the likelihood of degradation under corrosive high temperature conditions. Coating methods are needed that overcome the problems of lack of coating adhesion, durability and uniformity. The use of ion implantation as described herein as a method to treat heat resistant alloys with antifoulants for the inhibition of coke formation to accomplish this object has not been described in the prior art.
Accordingly, it is an object of the present invention to provide a treatment method for heat resistant alloys to reduce carburization and coke formation in thermal cracking processes by ion implantation of selected elements to form a treated surface layer that is uniform at the atomic scale, adherent to the substrate, and durable under corrosive high temperature conditions.
Objectives and advantages of the invention are:
(I) To provide a treated surface layer that inhibits carburization and coke formation rates;
(II) To provide a treated surface layer having a precisely controlled composition and depth profile;
(III) To provide a treated surface layer that is uniformly protective and relatively free of defects; and
(IV) To provide a treated surface layer that is adherent and possesses a low tendency to crack, peel, or degrade at high temperature.
In accordance with the objects of the present invention, there is provided a treatment method to obtain a precisely controlled surface composition on a substrate to form a uniformly protective surface layer that inhibits coking and carburization and possesses low tendency for cracking, peeling and degradation at the high temperature conditions typical of thermal cracking processes. Antifoulants selected from a group of primary elements consisting of aluminum, silicon, and chromium, or a combination thereof, and a group of secondary elements consisting of calcium, lithium, magnesium, cesium, hafnium, yttrium or zirconium, or combinations thereof are ion implanted into the surface of the metal substrate to form a durable oxide film. The oxide film consists primarily of Al2O3, SiO2, Cr2O3 or combinations thereof, and contains lesser concentrations of secondary elements that enhance the durability and coke-inhibiting quality of the treated surface layer.
It is a further object of the invention to provide a method for reducing carburization, oxidation, and the formation of coke on a metal object having a surface exposed to hydrocarbon at high temperature in a thermal cracking process, that includes:
a) providing ion implanting apparatus,
b) operating the apparatus to ion implant selected antifoulant or antifoulants into the metal object surface,
c) the metal object configured to have its ion implanted surface exposed to hydrocarbon at high temperature in the thermal cracking process.
Another object is to provide for generation of a plasma containing ions of the antifoulant or antifoulants, and exposing the metal object surface to the plasma, under vacuum conditions.
A further object is to provide the treated object in the form of a reactor pipe sized to flow a stream of hydrocarbon in a thermal cracking process furnace, and operation includes relatively moving the plasma and pipe, lengthwise of the pipe, to substantially uniformly treat the pipe bore with the plasma, and for a time period to achieve ion implant.
An additional object is to-provide antifoulants that comprise:
i) a primary element or elements selected from a first group comprising aluminum, silicon and chromium, and
ii) a secondary element or elements selected from a second group comprising calcium, potassium, lithium, magnesium, cesium, hafnium, yttrium or zirconium.
As will be seen the primary element or elements of the first group of elements are ion implanted at doses in the range 1xc3x971017 ion/cmxe2x88x922 to 1xc3x971018 ion/cmxe2x88x922; also the secondary element or elements are ion implanted at doses in the range 0 to 5xc3x971016 ion/cmxe2x88x922, and wherein the secondary element or elements are ion implanted pursuant to one of the following:
x1) subsequent to ion implantation of the primary element or elements,
x2) concomitant with ion implantation of the primary element or elements.
Yet another object is to generate and use one of the following for ion implantation:
i) directed beam ion implantation
ii) plasma source ion implantation
iii) plasma immersion ion implantation and deposition
iv) ion translation from an ion source onto the surface of said object.
These and other objects and advantages of the invention, as well as the details of an illustrative embodiment, will be more fully understood from the following specification and drawings, in which: